This invention relates to a method for molding bonded plastics material, in particular expanded or foamable plastics material in combination with rigid plastics material, such as expanded polystyrene with rigid sheet polystyrene.
For molding these materials, which requires the application of steam to sinter the expanded foamable plastics material, with its simultaneous adhesion to the rigid material, it is already known from Italian patent No. 1,185,507 to use a mold consisting of two half-mold which together define a molding cavity for housing the two materials, one of which, namely the more rigid sheet material, is firstly preformed in contact with only the lower half-mold, the other material then being molding onto the first by sintering in the closed mold.
In this method, the rigid sheet plastics material is retained at its end edges and then firefly hot-deformed by moving the relative half-mould beyond the plane of the sheet material, after which the material is made to adhere exactly to the half-mold by creating a vacuum between the half-mold and the material. The upper half-mold is then lowered and the expanded foamable plastics material is molded within the resultant cavity, steam being applied in pulses alternating with the application of vacuum, to enable the expanded material to sinter onto the more rigid material.
This known method and apparatus have the advantage of requiring only a single mold consisting of two half-molds movable relative to each other, with resultant constructional advantages. However, a vacuum application stage for preforming is still required, as otherwise it would not be possible to make the more rigid material adhere perfectly to the shape of the preforming half-mold, there being no corresponding counter-mold in that the one provided has a shape which corresponds instead to the part to be molded in expanded material. Vacuum application alone is not always able to produce the required shape, especially in the case of the more complex molds shapes, as in such cases it is difficult to cause the material to adhere to the mold at every point. To overcome this difficulty, preforming has to be effected in a mold separate from the main mold, however this complicates the apparatus because of the larger number of components and the consequent more complicated handling of these and of the material. A further drawback of the known method and apparatus is that those portions of the material which are peripherally clamped undergo inevitable alteration during molding and have mostly to be discarded as the cannot be recycled, this representing a substantial material wastage and posing disposal problems.